The invention relates to apparatus for the digitization of videofrequency signals used in television comprising on transmission a coder for said signals in digital form and on reception a decoder which restores them to analog form, as well as to transmitting equipment incorporating such an apparatus.
Certain known apparatus use a sampling frequency which is more than twice as high as the maximum videofrequency to be transmitted making it possible to obtain a good definition with an orthogonal structure of the points of the image, i.e. the latter are arranged in column form one above the other in conventional manner. They have the disadvantage of requiring a sampling frequency close to 12 MHz and therefore a considerable band width for the standard 625 lines, 5.5 MHz system.
For sampling the same signal, others use a lower sampling frequency, e.g. 8.875 MHz which is close to double the frequency of the subcarrier of the
system, i.e. less than double the maximum frequency to be transmitted (so-called sub-Nyquist sampling). Theoretically, this system does not make it possible to define a videofrequency above 4.4 MHz or in practice 4 MHz, which is inadequate. This defect can be obviated by selecting a staggered positioning of the image elements (samples) between one line and the next, leaving it up to the eye of the observer to subjectively carry out a type of interpolation (due to the natural tendency of seeking lines of points in images).
Although this system would appear to be satisfactory for moving images in the case of fixed images and particularly patterns, it is possible to observe a lack of definition in the horizontal direction. Moreover, the staggered structure leads to certain disadvantages during the production of special effects (electronic trick effects, split screen effects, shutters, etc).